I know several rpg rulesets well. I have hacked some of them extensively. I have designed a few of my own games whole-cloth. If I want to run a traditional rpg in a particularly setting or style and I don't know a ruleset that's right for it, I can probably hack something to fit.
Yet I'm still interested in learning new published rulesets. Not just high-concept storygames with very distinctive rules — I'm up for learning quite traditional rpgs. I'm doing this with Zweihander right now.
I find learning and using third party rules hard work, though. I can sink hours into learning, more hours into GM prep (slowly, because I'm clumsy and awkward the unfamiliar rules), and several hours at least into unsatisfying play (because it turns out I don't actually enjoy this weird new game). My experiences with new games is mixed, to say the least.
So why do it? Why bother? What's to gain?
I don't find those trivial questions. I've put my best answers and put them in a blog post
. I've hawked that around Reddit and rpg.net, and I've revised it based on the discussion there. Finally, I bring it to Storygames, where (if nothing else) I suspect I'll get responses from people whose relationships with GM'd-game-rules are as troubled as mine...